This invention generally relates to semiconductor protection circuitry and more specifically relates to an overdissipation and excessive current protection circuit for a transistor switch which automatically resets if the magnitude of the overdissipation is within a predetermined range when the cause of the overdissipation is removed.
It is often desirable to turn power on and off from a location which is physically remote from the source of power and the power sink. One reason for this is that it is undesirable to run significant current from the source to a location convenient for switching the current and then return the current to the load. A device which requires a small amount of current to switch a large amount of current can perform the remote switching function well.
Transistors are often employed to perform the remote switching function, for they switch relatively large amounts of current with a small control current. Transistors, however, are subject to damage or destruction if their current or power ratings are exceeded.
It has been shown that a field effect transistor (FET) switch can be protected from damage by excessive current by detecting a high drain-source voltage (V.sub.DS). This voltage may be input to added control circuitry for causing the FET to turn off. This switch may be reset by a capacitor-resistor charging network which cycles the FET on each time it is turned off as a result of excessive current. Protection for damage which may be caused by excessive power dissipation within the FET under near short circuit conditions is unaddressed.
The excessive dissipation damage problem is also commonly encountered in voltage regulator circuits. There, a sample of the current drawn by the load is typically fed back to regulator control circuitry such that upon overload, the current through the regulator is reduced. A short circuit may activate a regulator disable mechanism which protects the regulator and which may be manually or automatically reset.
In the typical voltage regulator protection circuit, a near-short circuit may cause the regulator device to dissipate power in excess of the maximum rated power despite the feedback signal. Circuits devised to fully protect the regulator device against excessive current and power dissipation have generally been complex.
Also, solid state circuit breaker circuits experience excessive current and power conditions. Solid state circuit breakers exist to interrupt current to a load when the current exceeds a predetermined magnitude. This magnitude need not be a short circuit. In order to reset the circuit breaker, an oscillator, which may be activated by a voltage differential across the tripped circuit breaker, is generally provided to generate pulses and cause the circuit breaker to reset. If the cause of the excessive current remains, the circuit breaker again trips and the cycle repeats until the cause is removed. The necessary detect and reset circuitry for present solid state circuit breakers has also been complex.
The present invention addresses the conditions of excessive current drain and overdissipation with a novel and uncomplicated circuit design to protect a semiconductor switch.